I have managed to find a few spare moments to get some practice in with Blenders keying nodes. and having a little bit of fun with the practice files being given away by Hollywood camera works. my first attempt wasn't too great as what looked like a good Key actually had a lot of holes in it where i had taken away more of the model than i was supposed to! but as I had a specific effect in mind it didn't turn out too bad after all!
my first effect was to use footage of smoke as a displacement map in order to distort the green footage, which i managed quite succesfully.
The nodes are a simple affair,
First i added two clips of smoke that i got from AEtuts.com and combinedthem using an alpha over node, then de saturated the result and then using a distort>scale node, set the clips to match the render size.
i then added my pre-keyed footage of the girl scaled it and then plugged it into a distort>displace node. i then took the smoke group and plugged it into the vector input of the displace node. i could then simply add a background using an alpha over node and call it done, but i decided to add more smoke and make her a bit ghostly and fade into the smoke and finally become more solid.
so the first thing i needed to do was to wash out the colour and fade that washed out colour into the full normal colour. I did this by adding an alpha over node and plugging the smoke group into the top colour socket. i then added a mix node and plugged the displace node into the top and the alpha over into the factor, to get the colour fade i added a time node, set it to 80 frames and adjusted the curve to a wierd little hump and plugged it into the factor of the alpha over. and finally i connected the output of the mix node of that little group to the alpha over that connects with the background.
that looked ok but i wanted more smoke over the top so i created a new group starting with a movie clip of more smoke fitted it to the render size and plugged it into another alpha over. this time into the bottom socket with the top colour set to black. this made the black parts of the footage transparent, i then duplicated the time node from earlier and plugged that into the new alpha over factor so the foreground smoke faded out at the same time as the colour change. a final alpha over was added and the smoke/background elements were plugged into the top socket and the foreground elements plugged into the factor and the output sent to the composite node for rendering. i then keyframed a few settings to get the transitions and diminish the distortion effect and that is it, done.
pretty simple and it gives a nice effect. of course, my editing leaves a bit to be desired, i could have worked a bit longer on that side of things but over all, not too bad!
for my next attempt i wanted to get a proper key with out having the issues with transparency that i got with the previous key
the only real secret to this is to pick a section of green that gives the best amount of transparency. most of the leftover carp was then removed by simply adjusting the clip black and clip white settings, almost everything else was colour correcting, although i did add a light wrap effect to blend the model into the scene a bit better. this is simply the matte result of the key inverted and multiplied by itself to give an outline which is then blurred and then used as a factor input for the background and foreground elements. a little bit of blur added to itself and some final colour correction and we have a nice result.
i think the camera movement kind of spoils things though, so, my next tutorial will be on match moving a background image to this footage. a kind of impossible thing as the whole point of the clip is that there are no tracking markers in the green screen...
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